Electrometers are instruments for measuring electric charge and thus, can be configured to measure either small voltages or currents. The present invention is an instrument for measuring small currents.
The input bias current for a solid state electrometer is a function of the operating temperature, and for field effect transistors, the usual devices used in a solid state electrometer, the input bias current doubles for every 10 degree centigrade increase in operating temperature. Since the input bias current for solid state electrometers is a measure of the lower limit of the electrometer's current measuring sensitivity, lowering the input bias current has the effect of permitting the measurement of lower currents. Normal laboratory operating temperature is twenty degrees centigrade; therefore, by reducing the operating temperature of the instrument to ten degrees centigrade the bias current is decreased by a factor of two. However, one of the problems associated with lowering the temperature of the electrometer is that the reduction of temperature leads to the formation of condensation on the instrument which in turn can lead to instrument failure or compromised readings of the measured current.
Another temperature problem arises when different parts of the instrument are operating at different temperatures. These temperature differentials lead to the formation of thermoelectric voltages which can lead to significant errors for a low voltage device such as an electrometer. One way to minimize the presence of the temperature gradients, and thus, the formation of thermoelectric voltages, is to place all of the junctions in close proximity to each other and then, to provide an effective thermal couple linking the junctions to a common heat sink. This arrangement provides a more uniform and stable temperature distribution and thus, a more sensitive current measuring capability.
Ambient room temperature variations can also limit the precision of measurements made by an electrometer. The electrometer circuit needed to implement the measurement of a small current is directly dependent on the presence of a high ohmic value resistor, one having resistance values greater than 10 to the 10 ohms, within the circuit. Typically, high value resistors have temperature coefficients of resistivity of at least 0.1 percent per degree centigrade; thus, the resistance of a high value resistor with a value of 10 to the 10 ohms could be changed by 10 to the 7 ohms with an increase of one degree centigrade in the ambient temperature surrounding the electrometer circuit. Therefore, by lowering and stabilizing the temperature of the electrometer circuit the resistance of the high value resistor will remain at a constant value resulting in a greater capability of the electrometer circuit to measure small currents.
Finally, the electrometer amplifier input offset voltage is also a temperature sensitive element which effects the current measuring capability of the electrometer circuit. For this element, a reduction in the temperature variation through the employment of a temperature controlled environment also results in an improved current measuring capability.
Accordingly, it is an object of this invention to provide an electrometer which is thermally cooled below the ambient room temperature to provide an improved small current measuring capability.
It is still a further object of this invention to provide a temperature controlled environment for the electrometer so as to provide a uniform and stable temperature environment as compared to the less stable conditions experienced in a uncontrolled, room environment.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.